Blocking of CTL-mediated killing by monoclonal antibodies to LFA-1 and Lyt-2, 3. II. Evidence that trypsin pretreatment of target cells removes a non-H-2 molecule important in killing

We sought additional evidence for an inverse relationship between functional CTL-target cell affinity on the one hand, and susceptibility of the CTL-mediated killing to inhibition by alpha LFA-1 and alpha Lyt-2,3 monoclonal antibodies on the other hand. Previously, we experimentally reduced affinity by pretreating the target cells with papain. This removed most of the class I H-2 antigens, had little effect on the ability of allospecific CTL to recognize and kill these targets, but dramatically reduced the initial strength of CTL-target cell adhesion, and increased by more than 10-fold the susceptibility of the killing to inhibition by alpha Lyt-2,3 and alpha LFA-1 MAb. In the present report, we find that pretreating the target cells with trypsin, like papain, does not significantly change the susceptibility of the target cells to killing by allospecific CTL in a 2-hr assay, and increases by about 10-fold susceptibility of the killing to inhibition by alpha LFA-1. Unlike papain, however, trypsin does not consistently increase blocking by alpha Lyt-2,3, does not remove class I H-2 antigens from the target cell, and does not substantially reduce the strength of initial CTL-target adhesion formation (estimated by post dispersion lysis after a 5-min conjugate-forming incubation). These results show a functional difference between LFA-1 and Lyt-2,3. Both papain and trypsin produced similar 10-fold increases in susceptibility to blocking by alpha LFA-1. In contrast, susceptibility to inhibition by alpha Lyt-2,3 was increased nearly 100-fold by papain, but was not consistently affected by trypsin. Thus, the above-mentioned inverse relationship holds for alpha Lyt-2,3 but not for alpha LFA-1. Our results are consistent with the hypothesis that Lyt-2,3 but not LFA-1 participates in recognition of class I H-2 antigens. Possibly LFA-1 participates in an adhesion-strengthening process that follows T cell recognition, and which may also be used by other LFA-1 expressing leucocytes in intercellular interactions. Finally, our results suggest (for the first time in the mouse system) that an unidentified non-H-2 "trypsin-sensitive counter blocking" molecule on the target cell plays an important role in CTL-target cell interaction.